It turns out it's really modprobe nbd max_part=16, otherwise max_part
defaults to, uhm, zero? really? and kpartx cannot create device mappings
because there are not enough (as in, not even a single one) partition devices
available.

At this point, however, kpartx did create some mappings connected to, uhm,
probably Ancient Beings from beyond spacetime, and because of those the device
is in use and cannot be removed, and unmapping doesn't work either because the
Ancient Beings from beyond spacetime are keeping the device busy by feeding on
it.